Likelihood of conception with a single act of intercourse: providing benchmark rates for assessment of post-coital contraceptives

doi:10.1016/S0010-7824(01)00191-3

Contraception

Volume 63, Issue 4, April 2001, Pages 211-215

https://doi.org/10.1016/S0010-7824(01)00191-3 Get rights and content

Abstract

Emergency post-coital contraceptives effectively reduce the risk of pregnancy, but their degree of efficacy remains uncertain. Measurement of efficacy depends on the pregnancy rate without treatment, which cannot be measured directly. We provide indirect estimates of such pregnancy rates, using data from a prospective study of 221 women who were attempting to conceive. We previously estimated the probability of pregnancy with an act of intercourse relative to ovulation. In this article, we extend these data to estimate the probability of pregnancy relative to intercourse on a given cycle day (counting from onset of previous menses). In assessing the efficacy of post-coital contraceptives, other approaches have not incorporated accurate information on the variability of ovulation. We find that the possibility of late ovulation produces a persistent risk of pregnancy even into the sixth week of the cycle. Post-coital contraceptives may be indicated even when intercourse has occurred late in the cycle.

Introduction

Post-coital contraceptives reduce the risk of pregnancy, although their degree of efficacy is unclear. Placebo-controlled studies are unacceptable, and researchers have instead relied on indirect estimates of what a woman’s probability of pregnancy might have been had she not been treated. We discuss methodological issues raised by these indirect approaches, and we use data from a prospective study to derive the (untreated) probability of pregnancy after a single act of intercourse on a given day of the cycle.

Section snippets

Materials and methods

Data are drawn from a prospective study of early pregnancy conducted in North Carolina [1]. Women (n = 221) who planned to become pregnant were enrolled at the time they discontinued use of any birth control. Women with known fertility problems were ineligible. Women were asked their usual cycle length, and whether their cycles were generally “regular” or “irregular.” (Exact questions provided in Appendix.) Women collected daily first morning urine samples, and they recorded menstrual bleeding

Results

The probability of pregnancy with one completely random act of unprotected intercourse was 3.1% in our data. Estimates can be substantially improved by including information on when intercourse occurred in the menstrual cycle. Fig. 1 shows the estimated probability of a clinical pregnancy with one act of intercourse during the cycle, estimated for each day of the cycle. (The dots show the probabilities calculated directly from the observed distribution of ovulation days, while the curve shows

Discussion

A single act of unprotected intercourse can occur when intercourse is unplanned, when there has been a contraceptive failure (as with a broken condom), or when there has been sexual assault. In 1960, Tietze estimated that the chance of pregnancy with one completely random act of intercourse was 2% to 4% [9]. More recently, Holmes and her colleagues reported that the probability of pregnancy after rape was 5% in a national sample of US women [10]. (Criminal sexual assault is presumably random

Acknowledgements

Valuable suggestions have been provided by Drs. Marcia Angle, Ruth Little, Andrew Rowland, Amy Sayle, and David Umbach. Data processing and graphs were provided by Dr. D. Robert McConnaughey.

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Original research articleLikelihood of conception with a single act of intercourse: providing benchmark rates for assessment of post-coital contraceptives

Abstract

Introduction

Section snippets

Materials and methods

Results

Discussion

Acknowledgements

Fertil Steril

Fertil Steril

Fertil Steril

Am J Obstet Gynecol

Fertil Steril

Contraception

Contraception

Fertil Steril

Incidence of early loss of pregnancy

N Engl J Med

Original research article
Likelihood of conception with a single act of intercourse: providing benchmark rates for assessment of post-coital contraceptives